Machine vision is useful in many applications, including reading indicia on semiconductor wafers during semiconductor manufacturing processes, as discussed in U.S. Pat. No. 6,870,949. In machine vision, there are many different types of cameras available. Camera systems include a device for image capture (like a CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) image sensor), a conversion from analog to digital pixel data (either located inside the sensor or not) and a method for transferring the data to a processing unit. In many cameras, including those with integrated lighting, they have separate data buses from the power source. This is seen with many serial protocols because the camera unit requires a separate power connection from a power supply, typically 12 to 24 volts (V).
Development of new powered bus architectures allows for the transfer of power along with the serial data. In this, the complexity of the remote device is lower due to the lack of additional power cables and connectors. However, the voltage level and the amount of current supported is mostly inadequate for lighting applications in machine vision.
Some serial bus standards have higher voltage and current ratings, like the IEEE (Institute of Electrical and Electronic Engineers) 1394 standard, which offers significantly more power and voltage than other standards like USB (Universal Serial Bus) 2.0. In the USB standard, the voltage offered is 5 V at 500 milliamps (mA). In comparison, IEEE 1394 offers 1.5 amps (A) at 12 V.
Machine vision cameras with integrated lighting require a large source of energy at a high voltage and high repetition rate to source the integrated lights at a particular repetition or frame rate. Presently available powered serial bus architectures, which allow for low cost and high performance data transfer, do not provide the voltage or current needed to source the lighting in the integrated camera unit.
The traditional methods of charging light circuits at a higher potential are to simply raise the bus voltage to the desired potential with a “boost” element, and then connect a bank of charge storage elements to the output of the “boost” element, as shown in FIG. 1, or to use a Xenon flash tube to create a minuscule amount of charge that is raised to a very high potential (e.g., 350 V). The problems inherent in the above approaches are particularly troublesome, and prohibitive in expense of circuit board surface area, component size, system cost, and short component lifetimes.
The simple boost conversion circuit 100 in FIG. 1 has two principal problems: (1) The current budget of 500 mA allowed on the USB bus is more than exhausted in the basic operation of the switching regulator circuit; and (2) the capacitive charge support is insufficient to provide for lighting pulses extending beyond tens of microseconds. Even a moderately large “ordinary” can capacitor (10 millimeter (mm) diameter×20 mm height), which is capable of withstanding 10 V, cannot be easily obtained in values much in excess of 1000 μF (microFarads). Another problem is that because the boost factor is two in this design, approximately twice the load current is sacrificed by being switched to ground.
Xenon flash charging circuits, which are common in disposable cameras, have the problem that a tiny amount of charge raised to a very high potential will significantly degrade the lifetime of the lighting elements, as this will produce intolerable thermal spikes.